使用“merge.conflictstyle diff3”时，如何使 git 合并产生更细粒度的冲突？

Question

I have a problem with an, in my opinion, unintuitive merge behavior of git (version 2.31.1). Let me demonstrate with a small toy repo.

git init
touch file1
git add file1
git commit -m "Initial commit"
git branch feature

Now I have a commit with an empty file and two branches, main and feature , pointing to that commit. Now I change the file in the main branch to

line1
line2
line3
line4
line5

and commit the change with

git add file1
git commit -m "Change file1"

Next, I go to the other branch

git checkout feature

change file1 there to

line1
line2
line
line4
line5

and again commit the change with

git add file1
git commit -m "Change file1"

The resulting commit tree looks like this:

* b1beb63 - Change file1 (HEAD -> feature)
| * 32ea83d - Change file1 (main)
|/
* 2952256 - Initial commit

Now I switch back to the main branch and merge feature into main .

git checkout main
git config merge.conflictstyle diff3
git merge feature

As expected, there is a conflict in file1 . However, the conflict looks like this:

<<<<<<< HEAD
line1
line2
line3
line4
line5
||||||| 2952256
=======
line1
line2
line
line4
line5
>>>>>>> feature

As you can see, git acts like the entire file is one large conflict. I would have expected it to look like this:

line1
line2
<<<<<<< HEAD
line3
||||||| 2952256
=======
line
>>>>>>> feature
line4
line5

which would be infinitely more user-friendly.

I found some threads where people had similar problems because of different line endings. This is not the case here as I edited the file in both branches on the same platform with the same editor.

My current solution for this type of merge is to find changes manually by doing a diff of the files between the two branches, ie git diff main:file1 feature:file1 . This works, but is rather annoying.

Is there any way to get the behavior I would expect in this merge? If not, is there any good reason why this is not possible?

Answer 1

tl;dr diff3 wants to see all three files in the merge: the two branches and their common ancestor. More aggressive stages of Git's conflict reduction can strip out the ancestor defeating the point of a diff3 conflict marker.

---

Git will normally refine its conflicts, as you say. UNLESS you're using merge.conflictstyle diff3 .

    if (style == XDL_MERGE_DIFF3) {
        /*
         * "diff3 -m" output does not make sense for anything
         * more aggressive than XDL_MERGE_EAGER.
         */
        if (XDL_MERGE_EAGER < level)
            level = XDL_MERGE_EAGER;
    }

level determines how aggressively Git refines the conflicts. If you're using diff3 it doesn't try very hard. Why? The original commit explains in excruciating detail. . Here is the summary as best I understand it.

Let's say you have a file like so.

1
2
3
4
5
6
7
8
9

And two branches both replace lines 5 and 6.

# branch1
1
2
3
4
A
B
C
D
E
7
8
9

# branch2
1
2
3
4
A
X
Y
E
7
8
9

The merge looks like this.

     branch1:     1234ABCDE789
                     |    /
                     |   /
     ancestor:    123456789
                     |   \
     branch2:     1234AXYE789

With normal 2-way conflict markers, Git would notice they both start with A and end with E and clip that out.

  A
++<<<<<<< HEAD
 +B
 +C
 +D
++=======
+ X
+ Y
++>>>>>>> feature
  E

This leaves out the ancestor from the conflict marker. This is fine for 2-way conflict markers, they only care about the two changed files. But you asked to see the conflict between between all three files, so that's what you get.

++<<<<<<< HEAD
 +A
 +B
 +C
 +D
 +E
++||||||| f8809d8
++5
++6
++=======
+ A
+ X
+ Y
+ E
++>>>>>>> feature

The extra conflict cleanup is incompatible with requesting diff3 style conflict markers.

Answer 2

Git 2.35.0 will have a new conflict style that behaves exactly as desired here called zdiff3 . It can be configured with git config merge.conflictstyle zdiff3 . Add --global to configure the conflict style globally.

I tested it with git version 2.35.0.rc0 and it produces the following result.

line1
line2
<<<<<<< HEAD
line3
||||||| 59df1b4
=======
line
>>>>>>> feature
line4
line5

Perfect!